<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            border: 1px solid #ccc;
        }
    </style>
</head>
<body>
      <button id="btn">点击将盒子变成红色</button>
      <div></div>
      <div></div>
      <div></div>
    <script>
         var bba=document.getElementById('btn')
         var ddv=document.getElementsByTagName('div')
             bba.onclick=function(){
                for(var i=0;i<ddv.length;i++){
                    ddv[i].style.background='red'
                }
             }
    </script>
</body>
</html>